1. Field of the Invention
This invention relates generally to paving machines and more particularly to apparatus for and methods of controlling the operation of such paving machines.
2. Discussion of the Related Art
Asphalt pavers for laying mats of asphaltic materials on roadways, parking lots or the like are well known in the art. A state of the art asphalt paver is a self-propelled unit capable of advancing along a designated course having a prepared base surface, and of depositing a paved layer of asphaltic material to a specified thickness, grade and slope on such base surface. The asphalt paver spreads and compacts asphaltic materials into a paved strip or mat of a certain width and thickness, and of an initial degree of compaction. After the paver establishes the desired slope and grade of the paved mat during the initial paving operation, the compaction of the paved material might typically be completed during a subsequent finishing operation with a compaction roller.
Smoothness and correspondence to specified grades and slopes of the paved mat are major considerations in assessing the quality of the paved mat as a product of an asphalt paving machine. Uniformity of compaction or consistency of the paved mat has a direct effect on the durability or ability of the paved mat to retain its specified grade and slope over time. Uniformity of compaction is consequently considered to be an inherent quality element of the paved mat as the final product. Lack of uniformity of compaction in the mat as it is laid down by a paver may result in surface waviness after final compaction of the mat by a vibratory roller, for example. Other defects in the paved mat may be caused by momentary lack or short supply of paving material which may cause surface voids to occur in an otherwise smooth mat. Stops or changes in paving speeds of the paver may also result in defects such as linear ridges or discontinuities in the surface transverse to the longitudinal extent of the paved mat.
Grade and slope of paved mats are determined by the height and transverse inclination of the trailing edge of a floating vibratory screed of the paver which is pulled along by and behind the tractor unit of the paver. The amount of paving material deposited under and compacted by the floating screed determines the final height and transverse slope of the screed and, hence, of the paved strip. The grade and slope control over the paved mat is exercised by changing the angle of attack of the lower, compacting surface of the screed with respect to its forward direction of travel, and by supplying a uniform amount of material at the leading edge of the screed. The angle of attack of the screed may be controlled by the angle of a pull arm on each side of the screed, in addition to an adjustable setting at the screed itself. The pull arms of the screed may be adjusted vertically independently of each other. The pull arms are moved jointly up or down to correct for errors in the grade of the pavement. Depending on the density or, conversely, the compactability of the material at the leading edge of the screed, the angle of attack of the screed may need to be changed to account for variations in material compression resulting from the vibratory paving or compacting action of the screed over the distance of its width in the direction of travel of the paver.
The lengths of the pull arms from the screed to a forward adjustment point, generally near the pull point by which the pull arms are coupled to the tractor unit of the paver, determine the precision with which an adjustment to the angle of attack or orientation of the screed can be made. Sensing any deviation of the pull arm from a desired grade at such forward position gives a recognition of any error at the forward position, but does not define the actual position of the screed. Sensing the actual position of the screed at the leading edge of the screed to correct positioning errors has been found to lead to a possible over-correction of errors in the screed angle of attack or angle of float. Such over-correction of minor deviations has resulted in oscillating paving thicknesses, and, hence, in unacceptable waviness of the paved strips laid down by the paver.
Similarly sensing the actual transverse slope of the screed at the screed and comparing it to a specified slope to generate a corrective control signal is known to cause overcontrol. Typical transverse slope controls are mounted across the pull arms of the screed just forward of the screed. The slope indicated by a single slope sensor mounted ahead of the screed is known, however, to introduce an error which reduces the accuracy of control over the screed slope. U.S. Pat. No. 4,925,340 shows a transverse slope control with dual slope sensors. A first slope sensor is located directly at the screed and senses the transverse slope of the screed. A second slope sensor is mounted across the screed pull arms and senses a slope across the left and right pull arms of a paver. Since a slope measured by the second sensor does not under all conditions accurately reflect the angular difference position of the pull arms, errors may be introduced into a control signal without knowing the actual skew or twist in the screed that may have been introduced by a corrective repositioning of the pull arms.
Various development efforts, over the years, have resulted in improvements pertaining to controlling the quality of the laid down pavements. U.S. Pat. No. 4,933,853 discloses an ultrasonic ranging transducer marketed by Polaroid. Such a transducer working with a Texas Instruments ranging module may be used as a digital distance measuring sensor. Having measured a vertical distance from the sensor to a grade reference, a control signal may be generated to control an angle of attack of a screed of a paver. Known slope controls permit an operator to set or simply dial in a specified percentage of transverse slope. The positioning of the slope control with respect to the screed may be critical as is the positioning of the grade sensor. Positioning the slope control directly on the screed has in the past been found to result in instability of control. The dual slope control shown in U.S. Pat. No. 4,925,340 uses an algorithm to interrelate signals from the two slope sensors, one being located directly on the screed of a paver and a second sensor at an intermediate position along the pull arms. An error signal of the slope of the screed is apparently integrated over a distance travelled by the paver and is further modified by adding the negative value of the slope signal of the intermediate sensor to arrive at a correction signal.
A banking slope in paving a curve is typically paved by requiring an operator to change the amount of slope at particular positions into and out of the curve to provide for an orderly increase of bank going into the curve and for an orderly decrease of bank going out of the curve into a subsequent straight stretch of pavement. A screed man may typically control and monitor the operation of the screed and correctness of any bank or slope of the paved strip separately of a paver operator. Though it may be desirable that an operator of the paver controls both the paver advance and the operation of the screed, a sole operator generally could not control the slope and grade and also steer the paver along the intended track of the curve and to maintain the correct grade of the pavement. However, even if two operators are used to separately control the paver and the screed, it is readily apparent, the operation of a typical state of the art asphalt paver is nevertheless complex and requires continuous attention to several process variables in order to produce a pavement of acceptable quality. Improvements in the control of asphalt paver operations are desirable to reduce margins of error, to increase reliability of operations and thereby reduce the cost of paving, and to produce pavements of highest possible quality.